Electricomechanical

I am actually confused about the method for selecting the suitable starter for the motors

Others here will be able to better advise you than me on that score - in the old days the choice (for AC motors) was little more than direct-on-line (DOL) or star/delta - but these days there are a myriad of electronic variable speed drives with so many variations you might end up just having to study some of the manufacturer's data. Certainly the size of the motor, the size mechanical load it might have to move when starting, the robustness of the supply will be just some of the factors to consider. In some case (e.g. lift motors) there might be requirements to re-generate electricity when slowing/stopping the motor which can add yet another tier of complexity.

Selecting the cable, breaker, contactors and over load relays!

I think the basic principles are similar though - generally fault an overload protection are usually dealt with separately. Cables are sized to cope with the motors normal running currents (plus possibly some extra for frequent starting) but protection at the distribution board only has to provide fault protection to the cable (e.g. dead shorts within the cable itself) - which usually means fuses or circuit breakers with an apparently much higher rating than the cables (there are even specific motor fuses with higher ratings than normal for their size).


Overload protection is usually dealt with at the motor end (where it is usually more convenient to reset) - traditionally using overload relays that were configured to suit the particular motor's normal current draw, these days more likely part of the variable speed drive.

     - Andy.

I am actually confused about the method for selecting the suitable starter for the motors

Others here will be able to better advise you than me on that score - in the old days the choice (for AC motors) was little more than direct-on-line (DOL) or star/delta - but these days there are a myriad of electronic variable speed drives with so many variations you might end up just having to study some of the manufacturer's data. Certainly the size of the motor, the size mechanical load it might have to move when starting, the robustness of the supply will be just some of the factors to consider. In some case (e.g. lift motors) there might be requirements to re-generate electricity when slowing/stopping the motor which can add yet another tier of complexity.

Selecting the cable, breaker, contactors and over load relays!

I think the basic principles are similar though - generally fault an overload protection are usually dealt with separately. Cables are sized to cope with the motors normal running currents (plus possibly some extra for frequent starting) but protection at the distribution board only has to provide fault protection to the cable (e.g. dead shorts within the cable itself) - which usually means fuses or circuit breakers with an apparently much higher rating than the cables (there are even specific motor fuses with higher ratings than normal for their size).


Overload protection is usually dealt with at the motor end (where it is usually more convenient to reset) - traditionally using overload relays that were configured to suit the particular motor's normal current draw, these days more likely part of the variable speed drive.

     - Andy.